How do MEMS pressure sensors work

MEMS - Micro-Electro-Mechanical Systems

MEMS are tiny components that combine logic elements and micromechanical structures in one chip. They can process mechanical and electrical information. Most of the elements are likely to be sensors and actuators, but also oscillators and filters. These mechatronic chips are mostly made of silicon. The structures can be smaller than a micrometer. Thanks to miniaturization, like semiconductors, they can be manufactured cheaply and in large quantities.

MEMS form an indispensable technical basis for modern, innovative solutions in electronics. Many applications cannot be economically implemented without MEMS. They make the use of electronic devices in the automotive, medicine, security and measurement technology, sports, logistics and entertainment sectors much more versatile, convenient and intelligent.
A typical application of MEMS is to measure acceleration. Or simply determining the horizontal or vertical position, as is known from smartphones by rotating the display. Here the screen display simply rotates with it.
In some everyday products, acceleration sensors are already an essential part. The acceleration sensors in hard drives recognize when the notebook falls. The read / write head is then automatically brought into the park position before the device hits the ground. In the parked position, the read / write head is prevented from hitting the surface of the rotating magnetic disk, scratching it and thereby destroying the data.
Many applications of MEMS are not really new. They were previously used in measurement, weapon and space systems. Expensive special modules were used for this.

Robustness, long-term stability and consistent product quality are often characteristics of MEMS. But also small size, lower price and low energy consumption are among their advantages.
The next step is nanoelectromechanical systems (NEMS), which can then also use quantum effects.

Manufacturing of MEMS

In general, MEMS, like semiconductors, can be manufactured cheaply and in large quantities. In fact, many MEMS can be produced using the standard CMOS manufacturing processes used in integrated circuits. But there are also MEMS that consist of two chips. For example, the one who has the sensor tasks and the other who serves as a signal amplifier or processor.
What you also need to know: Ordinary CMOS production systems are designed for planar structures. However, many MEMS have three-dimensional or deeply etched structures that require additional manufacturing steps, special materials or special etching processes.

MEMS applications

  • Cell phones that respond to movement.
  • Console games controlled by accelerating and rotating a controller.
  • Hard disks, the read head of which is automatically brought into the park position in the event of a fall.
  • Cameras that compensate for wobbling movements (image stabilization).
  • Pacemakers, respiratory and mobile EKG devices in medicine.
  • Micromechanical pumps that dispense drugs.
  • Airbags that deploy when the car is extremely slow.
  • Control signals for the electronic stability program (ESP) in cars.
  • Tablets that rotate the screen content when the device rotates.
  • Coolant supplies that fit into processor housings.
  • Mechanical switches with low operating power and long service life.

In MEMS, sensors and actuators are equipped with additional circuits for signal processing, automatic calibration and temperature compensation. This enables new types of applications, such as radio transmissions with an integrated power supply. For example through energy harvesting.

MEMS components

  • 3-axis inclination sensor
  • Pico projector
  • RF MEMS switch
  • Capacitance diode
  • Accelerometer
  • gyroscope
  • microphone
  • High frequency relay
  • electronic filters
  • Timer
  • optical switches
  • Coolant pumps

Example: MEMS microphone

In addition to a smaller housing volume, MEMS microphones contain additional functions. For example, a preamplifier adapted to the membrane, a voltage multiplier for generating a bias voltage for the membrane or an analog-digital converter.
There are also MEMS microphones that work on the capacitor principle or use the piezoelectric or piezoresistive effects of a silicon crystal. The membrane consists, for example, of polycrystalline silicon, metal layers or organic foils.

Example: multi-axis rotation rate sensor

The model for a micromechanical multi-axis yaw rate sensor is a gyro instrument, as it is used in aircraft or weapon systems. It usually weighs several kilos, is expensive and consumes a lot of energy. The same functionality can be found in smartphones, tablets and console controllers. They determine data for motion control and acceleration.
MEMS acceleration sensors or gyrators usually work according to the spring-mass or tuning fork principle. In the former, built-in signal amplifiers detect the deformation or movement of a spring via changes in capacitance or the piezo effect. With the tuning fork principle, magnetically excited comb structures vibrate, the deflection of which is recorded by sensors.

Structure of a MEMS acceleration sensor

The structure of an acceleration sensor based on the spring-mass principle is explained below. It should be noted that there are acceleration sensors that also work according to other principles.

The structure of this MEMS acceleration sensor consists of three stacked plates that are connected to one another by spiral springs. The outer plates are fixed and the middle one is movable. Your mobility is limited by the coil springs. This structure creates a series connection of two capacitors with variable capacitance. Changeable because the plate spacing of the two capacitors changes due to the middle plate. It is well known that the plate spacing influences the capacitance of a capacitor.

How a MEMS accelerometer works

If there is no movement or the acceleration is zero, then the plates are at the same distance from one another. If there is a movement, i.e. an acceleration, then the middle capacitor plate is pushed to the side. This creates a change in capacitance. This change in capacitance is proportional to the acceleration.
If a constant speed is maintained, there is still movement, but no more acceleration. The middle capacitor plate goes back to its original position.
The acceleration can be used to calculate the speed (integration). And from the speed (integral) the distance covered.

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